1. Field of the Invention
The present invention relates to a capacitor in which a stretch-oriented synthetic resin film is employed for a dielectric layer.
2. Brief Description of the Prior Art
As polyvinylidene fluoride films have a dielectric constant about three to six times as much as that of other plastic films and are superior in withstand voltage performance, they may provide a very small capacitor as compared with conventional capacitors.
Polyvinylidene fluoride films, however, are as poor as or poorer than other plastic films in impregnation with an insulating oil or the like so that they cannot utilize their properties fully even where they are employed for capacitors. The polyvinylidene fluoride films can little be penetrated and get wet with an impregnant due to physical and chemical properties on their surface. Even if they are impregnated with an impregnant, portions that are not impregnated with the impregnant would occur in the film so that they may cause a corona discharge at a high voltage, whereby the film is deteriorated and, as a result, the insulation of the capacitor would get rise to a damage.
In order to improve the poor impregnation of the plastic films for use with capacitor, the prior art technique generally employs a so-called composite dielectric element prepared by superposing the film or films with a sheet or sheets of insulating paper and winding the superposed material on themselves in a roll. In the case of polyvinylidene fluoride films, it may be possible to improve their impregnating ability to some extent by forming a composite element with insulating paper. In this case, however, the dielectric constant and the withstand voltage performance of the insulating paper is considerably lower than those of the polyvinylidene fluoride films so that the use of such composite dielectric element cannot provide a sufficient effect with respect to reduction of the capacitor size and improvement in withstand voltage performance.